Conventional processes for the diffusion coating of components made from heat-resistant alloys, such as Ni-, Co- or Fe-base alloys, include so-called powder pack processes. A process of this type is described, for example in U.S. Pat. No. 3,667,985, in which the component surfaces to be coated are 15 brought into contact with a donor powder including titanium and aluminum, to which an inert filler material and a halide activator are admixed, and is heated. U.S. Pat. No. 3,958,047 describes a powder pack process in which the metallic component is brought into contact with a donor powder which contains aluminum and chromium and is diffusion-coated by heating.
These processes are particularly suitable for coating the outer surfaces of metallic components, producing layer thicknesses of between 50 and 100 μm. However, these processes have inherent drawbacks when coating internal surfaces, and consequently the internal layer thicknesses which are achievable with relatively complicated geometrical forms with narrow gaps, tight angles or undercuts are limited and inadequate, generally being below 30 μm. A problem in this respect is that the donor powders have only a low ability to flow and therefore do not sufficiently fill the cavities. Moreover, after the coating, the donor powder can only be removed from the cavities with difficulty, and it is not possible to avoid leaving residues, and also the donor powder sinters to the surfaces.
The abovementioned drawbacks of the powder pack processes can in part be eliminated using so-called gas diffusion coating processes. One process of this type is described in U.S. Pat. No. 4,148,275, in which a powder mixture which contains, for example, aluminum is arranged in a first chamber and the metallic components to be coated are arranged in a second chamber of a vessel. The coating gas is generated by heating the powder and, using a carrier gas, is deposited on the outer and inner surfaces of the components to be coated. However, these gas diffusion coating processes have the drawback that the devices for carrying out the process, such as, for example, for the forced guidance of the coating gases, are complex and expensive compared to those used for the powder pack processes. Furthermore, the internal layer thicknesses which can be achieved are limited, since the coating gas or the donor metal gas is depleted on its route through the cavities of the component and a layer thickness gradient is produced along the length of the cavity. Since process conditions mean that the layer thickness of the outer coating is greater than that of the inner coating, the service life of the component is limited on account of the thinner internal coating.
U.S. Pat. No. 4,208,453 describes a process for the diffusion coating of the inner and outer surfaces of components, such as gas turbine blades, in which a powder mixture comprises 10% of chromium donor powder with a particle size of from 10 to 20 μm and 90% of alumina granules with a particle size of from 100 to 300 μm. In addition, a metal halide is added as activator. This process does not deal with measures for increasing the layer thickness in cavities of complicated geometry.
German Published Patent Application No. 30 33 074 describes a process for the diffusion coating of the inner surface of cavities, in which a metallic workpiece can be coated with an aluminizing diffusion powder mixture comprising 15% of aluminum powder with a particle size of 40 μm and 85% of alumina powder with a particle size of approximately 200 to 300 μm, as well as an NH4Cl powder.
U.S. Pat. No. 5,208,071 describes a process for aluminizing a ferritic component with an alumina slurry, followed by heat treatment, the slurry including at least 10% by weight of chromium, at least 10% by weight of inert filler material, at least 12% by weight of water, a binder and a halogen activator, and finally the coated ferritic component is heat-treated. The process technology involved means that the use of a slurry differs significantly from a powder pack coating process.
British Published Patent Application No. 2 109 822 describes a metal diffusion process with which diffusion coatings can be produced more quickly than in the powder pack process, the coating powder being in loose form and being kept in contact with the component to be coated, in particular including with its internal surface, by mechanical means during the heating. The composition of the coating powder may comprise 10 to 60% of chromium powder, 0.1 to 20% of chromium halide and alumina.
It is an object of the present invention to provide an improved powder pack process so that the layer thicknesses of the internal coating are sufficiently great even in the case of cavities with relatively complex geometries.